Determination of the height of the surface of a fluid column

ABSTRACT

The invention concerns a method for reducing the effect of a rough sea ghost reflection in marine seismic data. According to the invention, the method comprises the steps of: providing one or a plurality of pressure sensors sensitive to frequencies below about 1 Hz;—using said sensor(s) to receive and acquire pressure data in a frequency band comprised between about 0.03 and about 1 Hz;—recording said data; and—processing said data to provide information about the sea-height above the or each sensor. The or each sensor may be a seismic sensor that can acquire seismic data substantially simultaneously with the acquisition of the pressure data in a frequency band between about 0.03 and about 1 Hz.

The present invention relates to a method of and a system fordetermining the height of a surface of a fluid column above a sensor.The method may be of use in, for example, marine seismic dataacquisition.

Marine seismic data acquisition may be achieved by seismic vesselstowing a seismic source and/or one or a plurality of instrumented cablespacked with sensors. In conventional marine surveys, those instrumentedcables, called streamers, are towed approximately horizontally at adepth between about 5 and about 50 meters.

FIG. 1 is a schematic diagram showing the various events that can beacquired by a towed streamer “STR” and recorded in a seismogram. Theseevents are shown and labelled according to the series of interfaces theyare reflected at, said interfaces being referenced “S” for the rough seasurface, “W” for the sea floor and “T” for a target reflector. The starsindicate seismic sources and the arrowheads indicate the direction ofseismic wave propagation at the receiver. Events comprising an “S” arereflected at the rough sea surface and are called ghost events.

Ghost events are an undesirable source of perturbations, which affectthe response of a receiver and the shape of the source pulse, henceobscuring the interpretation of the desired up-going reflections fromthe earth's sub-surface.

The effect of the rough sea is to perturb the amplitude and arrival timeof the sea surface reflection ghost and to add a scattering coda or tailto the ghost impulse. FIGS. 2A and 2B compare two typical rough seaimpulse responses to a flat sea impulse response. Those responses, whichare simulated, are computed at a single point located at a nominal6-meter depth below the mean sea level. In one rough sea response, thereis an increase in both the ghost arrival time and amplitude. In theother response, there is a decrease. The pulse shape is also perturbed.There is a trailing coda at later times resulting from scattered energyfrom increasingly distant parts of the surface which gives rise toripples on the amplitude spectra. The spectral ripples in the 10-80 Hzregion can be a significant source of error.

FIG. 3 is a simulation, which illustrates how the rough sea effect candegrade a seismic image. It also illustrates how that degradation may besignificant, in particular, for time-lapse surveys, wherein seismicimages are made at different times, for example, one year apart, inorder to evaluate, notably, the change of the oil level of a reservoir.The panel on the bottom left shows a section of a subterranean earthmodel. The panel on the top left is a representation of the seismic datathat can be acquired from this model, with a flat sea, and the panel onthe top right is a representation of the data that can be acquired fromsaid model, with a 2 m Significant Wave Height (SWH) rough sea, in atime-lapse survey. Finally, the panel on the bottom right is adifference between these two representations multiplied by a factor of2, which has been caused by the roughness of the sea. It clearly appearsthat the rough sea effect can degrade the seismic image and that thisdegradation can be significant and may mask a genuine difference.

Various patent applications disclose methods for correcting or reducingthe rough sea effect in seismic data. This is the case, in particular,of the methods disclosed in the applications published under the numbersWO 00/57206 and WO 00/57207. Normally, the seismic signals received bythe seismic sensors are filtered before being recorded so that databelow about 3 Hz are rejected. Some ghost correction methods depend onknowing the height of the sea surface as a function of time, above eachsource or receiver. The sea surface shape is then extrapolated away fromthe sensor. This extrapolation may simply be a plane passing through themeasured height or may be more elaborate. Nevertheless, none of thesemethods discloses how the height of the sea surface may be measuredusing, in particular, streamers of the state of the art.

Considering the above, one problem that the invention is proposing tosolve is to carry out an improved method for determining the height ofthe surface of a fluid column.

The proposed solution to the above problem is defined in claim 1.

The time-varying shape of the sea surface gives rise to pressure waves,and these sea surface pressure waves occupy the frequency band comprisedbetween about 0.03 and 0.5 Hz. However, because of the movement of thesensors relative to the waves, said frequency band is extended to about0.03 to 1 Hz by the Doppler effect. According to the invention, the dataof the 0.03-1 Hz frequency band are not only received and acquired bythe sensors, but they are also recorded and processed to provide anestimate of the sea surface elevation above each sensor.

Further aspects and preferred features of the invention are defined inthe other claims.

The invention will be better understood in the light of the followingdescription of non-limiting and illustrative embodiments, given withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing the various events that may bereceived by sensors of a towed streamer;

FIGS. 2A and 2B show typical perturbations caused by a rough sea ascompared to a flat sea;

FIG. 3 shows a model and three seismic images of said model, whichillustrate the degrading effect of a rough sea;

FIG. 4 illustrates the smoothing effect for various depths of sensors;

FIGS. 5A and 5B show the Q raw data that may be acquired and recordedaccording to the invention;

FIG. 6 shows depth filter curves for two different sensor depths and twodifferent sea depths and compares these curves with a Pierson Moskowitzspectrum for a SWH being equal to 4 m;

FIG. 7 shows an apparatus according to the invention; and

FIG. 8 shows a seismic surveying arrangement according to the invention.

The invention will be described with reference to an embodiment in whicha plurality of pressure sensors, sensitive in the 0.03 Hz to 1 Hzfrequency range are provided on an instrumented cable, in this example aseismic streamer, that is towed through the sea by a vessel. However, inother modes for carrying out the invention, said the pressure data inthe 0.03 Hz to 1 Hz frequency range may be acquired by a sensorsdisposed on a plurality of streamers, by sensors disposed on one or moreOcean Bottom Cables (OBCs) laid on the seafloor, or by one or moresensors disposed adjacent to a seismic source.

A seismic streamer has a length typically of a few kilometres. Accordingto this embodiment of the invention, a seismic streamer is provided withone or, preferably, a plurality of sensors capable of recording a streamof low frequency pressure data. Typically each sensor will digitallysample the pressure at regular time intervals, with the interval betweensuccessive sampling operations being known as the “sampling interval”.

In a particularly preferred embodiment of the invention, the sensor, orat least one of the sensors if there are more than one is advantageouslya seismic sensor, that is to say a sensor that is also capable ofreceiving and acquiring seismic data. In a particularly preferredembodiment the or each sensor may be a seismic pressure sensor such as ahydrophone. Alternatively the or each sensor may be a comprised in amulti-component seismic receiver, for example, a 4C receiver that hasgeophones for measuring particle velocity in three directions (x, y andz) and a pressure sensor such as a hydrophone. Thus in this embodiment asensor of the streamer acts both as a sensor for receiving and acquiringpressure data in the frequency range from 0.03 to 1 HZ and as a seismicsensor for acquiring seismic pressure data—in this embodiment, theseismic pressure sensors that are ordinarily disposed on a seismicstreamer are themselves used to acquire the pressure data in the 0.03 Hzto 1 Hz frequency range, so that this embodiment of the invention doesnot require additional pressure sensors to be provided on the streamer.

Hydrophones are sensors comprising a piezo-electric device in order tomeasure pressure variations in a certain frequency domain. In aconventional streamer hydrophones are distributed singly or in groupsalong the length of the streamer, at regular intervals. For example,groups 12.5 m long and containing 12 hydrophones may be provided, orgroups 6.25 m long and containing 6 hydrophones may be provided. Thehydrophones or the hydrophone groups are decoupled one from the othersso that all pressure data that they acquire are transmitted, afteranalogue-to-digital conversion and multiplexing, via optical fibres,wires or other data transmission devices, along the streamer, to acomputer onboard the towing vessel where they are recorded.

An example of commercially available streamer is exploited under theappellation “Q” by the company named WesternGeco. This streamer isprovided with a plurality of decoupled hydrophones that can be used assensors according to the invention. The invention is not, however,limited to use with this particular streamer.

Typically a hydrophone or other pressure sensor disposed on a streameris provided with, or is associated with, a digital low-cut filter, whichnormally blocks low-frequency pressure data, for example blocks pressuredata in the frequency range below 3 Hz. Data at frequencies below 3 Hzare not normally of interest in a seismic survey, since seismic data aretypically contained in approximately the 3 to 80 Hz frequency band. Thelow-cut filter may be applied either at the acquisition of the seismicdata or later during processing of the data. In order to use aconventional hydrophone provided on a streamer as a pressure sensor forobtaining low frequency pressure data in the 0.03 to 1 Hz range it isnecessary to disable the associated low-cut filter. Once the low cutfilter is disabled, the hydrophones are not only able to receive andacquire seismic pressure data, which are contained in approximately the3 to 80 Hz frequency band, but they are also able to receive and acquirepressure data at frequencies below 3 Hz which are not, by themselves,seismic data since they do not relate to the sea floor subsurface. Oncethe low-cut filter has been disabled, each pressure sensor is able tomeasure and acquire low frequency pressure data from which the height hof the sea surface above the sensor may be derived. In the case wherethe low-cut filter is applied during processing of the acquired data,the data received and acquired at frequencies below 3 Hz have a dynamicrange high enough to permit their further use according to the inventiononce the low-cut filter has been disabled.

For a flat sea, the pressure below the sea surface is given by:P ₀ =ρ g z  (1)where P₀ is the hydrostatic pressure sensed by the sensor, ρ is thedensity of the water, g is the acceleration due to gravity and z is thedepth of said sensor below the Mean Sea Level (MSL). However, for arough sea, a pressure sensor detects a pressure, which is not simplyrelated to the height of the sea immediately above it (D. J T Carter, P.G. Challenor, J A. Ewing, E. G. Pit, M. A. Srokosk and M J Tucker,“Estimating Wave Climate Parameters for Engineering Applications”,Offshore Technology Report OTH 86 228, 1986 (Carter et al.)). Assumingthat the system can be treated as linear and that the effect ofdifferent sea surface waves may be superimposed, the dynamic part of thepressure sensed by a pressure sensor is:p=ρ g h cosh(k(d−z))/cosh(kd)  (2)wherein p is the dynamic part of the pressure, k is the wavenumber ofthe sea surface wave equal to 2π/λ where λ is the wave length, h is theupward displacement of the sea surface directly above the sensor,relative to MSL, and d is the ocean depth relative to MSL.

For an infinitely deep ocean, the equation (2) simplifies to:p=ρ g h exp(−kz)  (3)

It appears from equation (3) that pressure sensors are particularlysensitive to the variations in the sea height that have smallwavenumbers, k compared with their depth z. Variations in the sea heightthat show large wavenumbers, k, and, therefore, short wavelengths, λ,are smoothed and are detected with reduced amplitude. The smoothingeffect is disclosed by Carter et al.

Equation (3) may be modified if desired to take account of non-linearterms, viscosity and surface tension. The former is particularlyimportant in the case of sea height estimation for breaking waves.

As shown in the FIG. 4, wherein depths are determined using a pressuresensor mounted at 2, 4 and 8 m below MSL and compared with the trueheight profile of a 4 m SWH, the error is not insignificant and thedeeper the sensor is deployed, the more the height reading is smoothed.The reduction in amplitude at short wavelengths is preferably correctedfor in the processing of the data.

It is known that the sea surface waves occupy the part of the frequencyspectrum comprised between about 0.03 and about 0.5 Hz. Although the seasurface waves occupy the frequency range 0.03-0.5 Hz, however, thisfrequency range is extended to 0.03 to 1 Hz owing to the longitudinalmovement of the sensor in the direction of the vessel and relative tothe wave movement, according to the Doppler effect.

FIGS. 5A and 5B show an example of raw pressure data that were receivedor acquired, recorded and used by a Q streamer. The low cut filtersassociated with the pressure sensors on the streamer, which areconventional 3 Hz digital low-cut filters, had been disabled. In thiscase, the vessel towing the Q streamer on which the pressure sensors aremounted is sailing into the wind. In FIG. 5A, the raw data are shown.The horizontal axis shows the first 400 meters of the streamer, whereasthe vertical axis shows the time in seconds. The diagonal lines in thedata correspond to ocean waves travelling along above the streamer. FIG.5B shows the fk-spectrum of the data of FIG. 5A. The branch that goes ofto the left and ends somewhere at 0.5 Hz corresponds to the wavespassing over the streamer. The striped pattern is a Gibbs typephenomenon that can be avoided by properly scaling the data from thedifferent streamer segments.

Therefore, according to the invention, the sensors, sensitive tofrequencies below about 1 Hz, are used to receive and acquire frequencydata relating to the sea waves in a frequency band comprised betweenabout 0.03 Hz and about 1 Hz. The data are transmitted, from thesensors, to a computer memory onboard the towing vessel. The dataacquired by a sensor, or group of sensors are recorded and thenprocessed for determining the height of the sea surface above the sensoror group of sensors.

In a preferred method of processing the low frequency pressure data todetermine the height of the sea surface above the sensor, the heightsthat are obtained directly from the pressure measurements and recordedare corrected to take into account the movement of the sensor in thedirection of the vessel. This may be done by interpolating themeasurements to a line of points that are stationary in the water. Ifthe water is moving over the ground, for example because of a tidalaction, the data may also be interpolated to the frame of the water, notto the frame of the land, because it is in the water frame that thepressure waves propagate.

Once the sensor's motion has been corrected for, the pressuremeasurements are preferably further corrected for the smoothing effectcaused by the depth of the sensor. The correction factor is derived fromthe above-referenced equation (2) for each k component of the surfacewavefield. The k-spectrum of the surface is derived from the frequencyspectrum of the pressure data and knowledge of the dispersion relationof the surface waves:ω² =g k tanh(kd)  (4)where ω is the angular frequency of the surface wave equal to 2π/τ whereτ is the wave period equal to 1/f where f is the frequency in Hz, k isthe surface wavenumber and d is the ocean depth relative MSL. For aninfinitely deep ocean, this reduces to:ω² =g k  (5)So, in the deep water limit, the equations (3) and (5) give:p(ω)=ρg h(ω) exp(−ω² z/g)  (6)which is the correction filter that may be applied according to theinvention, for an infinitely deep ocean. The data from each receiver canbe deconvolved without using data from the other receivers.

It is noted that the low pass filter exp(−ω²z/g) can be removed bydeconvolution of the h(t) signal.

For the case of finite ocean-depth, equations (2) and (4) are combinednumerically to define the filter. However, the effect of ocean depth isnot large for oceans that are 50 metres deep or more. FIG. 6 shows thedepth filter curves for two different sensor depths, 6 m and 12 m, andtwo ocean depths: infinite and 50 m. In addition, the 4 m SWHPierson-Moskowitz isotropic ocean wave spectrum is plotted to show theactive part of the spectrum. Each filter curve splits in two at lowfrequencies corresponding to an infinite ocean depth and 50 m oceandepth. Over the sea wave spectrum bandwidth the effect of ocean depth issmall. It can be seen that the effect of the finite ocean depth on thesensor filter is small, being at most a few percent over the active partof the spectrum.

Thus, the invention provides a determination of the height of the seasurface above the or each pressure sensor. In an embodiment in which theor each pressure sensor is a seismic sensor, the invention thereforeprovides local sea-height data for the seismic sensors, since itprovides a determination of the height of the sea surface above the oreach seismic sensor.

Furthermore, as noted above, each pressure sensor will typicallyrepeatedly sample the pressure. The data acquired in successive samplingoperations may be processed as described above to provide adetermination of the variation with time of the height of the seasurface above the seismic sensor.

The local sea-height data obtained for each pressure sensor has manyapplications.

For example, the height of the sea surface above each pressure sensorpermits the reconstruction of the profile of the sea surface. This maybe done using, for example, an extrapolation of the time varying surfaceelevation along the line of the streamer. It may alternatively beachieved by a statistical interpolation method such as that thatsuggested by J. Goff for determination of the seafloor profile.

Once the sea surface has been reconstructed, the reflection response ofthe sea-surface can be computed. This can be done, for example, byKirchhoff integration, by a Lax-Wendroff technique, or by any othersuitable technique. A deconvolution operator may then be calculated andapplied to seismic data acquired at the same time as the pressure dateused to obtain the profile of the sea-surface to correct the seismicdata for the effects of the time-dependent height of the sea surface.For example, the estimate of the time-dependent height of thesea-surface may be used to reduce the effect of rough sea ghostreflections in the seismic data. The quality of the seismic images thatare obtained is thereby improved.

The sea height data obtained from the low frequency pressure data may beused to correct seismic pressure data obtained by the same sensor forthe effect of the time-dependent height of the sea surface. It may alsobe used to correct other seismic data—for example, if the low frequencypressure data is acquired by a pressure sensor located in a 4C seismicreceiver, the sea height data obtained from the low frequency pressuredata may be used to correct, for example, particle velocity dataacquired by a geophone in the 4C receiver as well as to correct seismicpressure data.

The sea-height data obtained by a method of the invention mayalternatively be used to determine the state of the sea-surface, inparticular to estimate the wave height, and this is known as “sea stateQC”. Sea state QC is currently carried out by making a visualobservation of the sea surface and assigning a numerical value to thewave height. According to the present invention, however, the waveheight of the sea surface may be determined from the local sea heightdata obtained from the low frequency pressure measurements, or from there-constructed profile of the sea surface derived from the local seaheight data. This provides a more accurate determination of the waveheight than can be obtained by visual observation.

The local sea height data provided by the present invention may also beused to ensure that the streamer is correctly levelled. Generally, it isdesired for a streamer to be substantially level (horizontal in thewater) during a seismic survey. Local sea height data may be obtainedaccording to the invention after a streamer has been placed in thewater, and this will show whether the streamer is level in the water,and also whether the streamer is at its desired depth below the mean sealevel. The streamer, or one or more segments of the streamer, can beadjusted as necessary, and once the local sea height data indicates thatthe streamer has been adjusted to be level and at the correct depth thestreamer is then ready for seismic data acquisition.

The local sea height data may be monitored during a survey to ensurethat the streamer remains level and at its desired depth during asurvey. For example, if the local sea height data showed that the depthof one section of the streamer was increasing, whereas the depth ofother sections of the streamer has remained substantially unaltered,this would strongly suggest that a leak had occurred in one sectionwhich was sinking as a result of the intrusion of sea water.

As noted above, in one preferred embodiment of the invention the lowfrequency pressure date is acquired using a seismic pressure sensor suchas a hydrophone. This allows the low frequency pressure data to beacquired simultaneously with the seismic data This in turn allows thedetermination of local sea height data for times at which seismic datawere acquired, for example for use in de-ghosting the seismic data.Where low frequency pressure data and seismic data are acquired togetherin this way, the low frequency data are preferably received and acquiredsimultaneously with the seismic data and over at least the same timeperiod as the seismic data. For example, the low frequency pressure datamay be acquired during a period from twenty seconds before the start ofseismic data acquisition to twenty seconds after the end of seismic dataacquisition.

It should be noted that, in practice, a hydrophone has an inherentlow-cut filter (in addition to the digital low-cut filter referred toabove). A hydrophone acts as a capacitor at low frequencies, and theelectrical wiring carrying the output signal from the hydrophone willact as a resistance; furthermore the signal from a hydrophone isgenerally fed to a voltage amplifier, and this will have an inputimpedance. The hydrophone capacitance and the circuit resistance willact as a low cut filter. This low-cut filter may well attenuate theamplitude of the hydrophone output for pressure waves in the frequencyrange 0.03 to 1 HZ. In order to determine the local sea heightaccurately, a correction must be made for the effect of this low-cutfilter, and this known as “backing off” the filter. If the hydrophonecapacitance and the wiring resistance are determined, the acquired datacan be corrected for the effect of the inherent low-cut filter.

A conventional streamer is generally provided with depth sensors, inaddition to the seismic sensors. These are generally hydrostaticpressure sensors, which determine the hydrostatic pressure atfrequencies below about 0.02 Hz; the depth of the sensor is obtainedfrom the measured hydrostatic pressure, according to equation (1).(Depth sensors are generally pressure sensors with a pressure to depthconversion based on (nominal or calibrated) water density and airbarometric pressure, and do not directly measure depth.) Theseconventional depth sensors may be used to check the quality of, orcalibrate, the low frequency pressure data acquired by a hydrophone.Such a check is useful, since the noise content of a hydrophone outputcan be significant at low frequencies. The calibration provided by adepth sensor operating at 0.02 Hz or below may well extend well beyondthe hydrophones located closest to the depth sensor, because the verylow frequencies at which the depth sensor operates correspond to surfacewaves having a very large wavelength.

In the description of the above embodiment, the pressure data in thefrequency range 0.03 Hz to 1 Hz is acquired using a seismic sensor. Theinvention is not limited to this, however, and it is possible for thepressure data in the frequency range 0.03 Hz to 1 Hz to be acquiredusing one or more separate sensors provided specifically for thatpurpose. For example, in such an embodiment a seismic streamer could beprovided with one or more sensors, additional to the streamer's seismicsensors, for acquiring pressure data in the frequency range 0.03 Hz to 1Hz. The additional sensors could be any pressure sensor that is capableof acquiring pressure data in the 0.03 to 1 Hz frequency range. In thisembodiment the streamer has a first set of one or more sensors foracquiring the low frequency pressure data and a second set of one ormore sensors for acquiring seismic data—the streamer's seismic sensorsacquire seismic data, and the additional sensors on the streamer acquirelow frequency surface wave pressure data.

Where the output from such additional low frequency pressure sensors isto be used in de-ghosting seismic data acquired by the seismic sensorsof the streamer, each low frequency pressure sensor is preferablysubstantially co-located with a respective seismic sensor. Each lowfrequency pressure sensor is preferably placed coincident with or withinabout 3 m of the seismic receiver to be corrected. Furthermore, the lowfrequency pressure data are preferably received and acquiredsubstantially simultaneously with the seismic data and over at least thesame time period as the seismic data. For example, the low frequencypressure data may be acquired during a period from twenty seconds beforethe start of seismic data acquisition to twenty seconds after the end ofseismic data acquisition.

Although the invention has been described above with particularreference to a seismic streamer, the invention is not limited to thisbut may be applied to any seismic receiver array. If the receiver arrayincludes seismic pressure sensors the invention may be effected by usingthe seismic pressure sensors to acquire the low frequency pressure data,and/or by using one or more additional low frequency pressure sensors toacquire low frequency pressure data If, on the other hand, the receiverarray does not include seismic pressure sensors, the invention may beeffected by using one or more additional low frequency pressure sensorsto acquire low frequency pressure data.

In principle, the invention may be effected using a single low frequencypressure sensor. This will, however, provide only limited informationabout the sea-height (namely, a single value of the sea-height above thesensor). The use of a plurality of low frequency pressure sensor ispreferable, since this provides information about the height of thesurface of the fluid column above each of a plurality of sensors and soallows generation of a profile of the sea surface from the informationabout the height of the surface of the fluid column above each of theplurality of sensors, for example by interpolation of the sea-heightbetween these locations.

The invention has been described above with reference to one or more lowfrequency pressure sensors disposed on a receiver array. The inventionis not limited to this, however, and may be applied to a marine seismicsource array by providing one or more pressure sensors sensitive in the0.03-1 Hz frequency band on the source array, with each sensor beingassociated with a seismic source or with a respective seismic source.The output from the sensors can be processed as described above toprovide the local sea-height above the or each sensor. This may be used,for example, to correct the rough-sea ghost response of the source, inwhich case each low frequency pressure is preferably substantiallyco-located with its respective source, for example being placedcoincident with or within about 3 m of the seismic source to becorrected.

It should be noted that the processing required to determine thelocalised sea height above a sensor provided in or on a source array isnot exactly the same as for a sensor provided in or on a receiver array.A source array is generally suspended from a float and so is positionedat a constant distance beneath the sea surface—i.e., the source arraymoves up and down as the height of the sea changes. This movement of thesource array introduces a Doppler shift and this must be accounted forin processing data acquired by a sensor disposed on the source array.(In contrast, a streamer is generally maintained at a constant “depth”independent of sea height/swell by depth control devices.).

FIG. 7 is a schematic block diagram of an apparatus 1 that is able toprocess low frequency pressure data acquired by a method according tothe present invention to determine the local sea-height above the oreach sensor. In a preferred embodiment, the apparatus 1 is further ableto process seismic data using the local sea-heights to attenuate theeffect of ghost reflections in the processed seismic data.

The apparatus 1 comprises a programmable data processor 2 with a programmemory 3, for instance in the form of a read only memory (ROM), storinga program for controlling the data processor 2 to process seismic databy a method of the invention.

The apparatus further comprises non-volatile read/write memory 4 forstoring, for example, any data which must be retained in the absence ofa power supply. A “working” or “scratch pad memory for the dataprocessor is provided by a random access memory RAM 5 An input device 6is provided, for instance for receiving user commands and data. One ormore output devices 7 are provided, for instance, for displayinginformation relating to the progress and result of the processing. Theoutput device(s) may be, for example, a printer, a visual display unit,or an output memory.

Sets of data for processing may be supplied via the input device 6 ormay optionally be provided by a machine-readable data store 8.

The results of the processing may be output via the output device 7 ormay be stored. The program for operating the system and for performingthe method described hereinbefore is stored in the program memory 3,which may be embodied as a semiconductor memory, for instance of thewell known ROM type. However, the program may well be stored in anyother suitable storage medium, such as a magnetic data carrier 3 a (suchas a “floppy disk”)s or a CD-ROM 3 b.

FIG. 8 shows an embodiment of seismic surveying arrangement according tothe present invention. The seismic surveying arrangement comprises asource array, indicated generally by 10, that contains one or moreseismic sources and is suspended below the sea surface from a surveyvessel 11. The seismic surveying arrangement further comprises areceiver array. This is shown as a streamer 12 that is also towed fromthe survey vessel in FIG. 8, but the receiver array could be any otherreceiver array such as, for example, a plurality of streamers or anOcean Bottom Cable. A plurality of seismic receivers 13 a, 13 b, 13 ceach of which consists of or includes a seismic pressure sensor such as,for example, a hydrophone, are provided on the streamer. The seismicdata acquired by the seismic receivers 13 a, 13 b, 13 c are passed, viaoptical fibres, wires or other data transmission devices, along thestreamer, to first processing and/or recording equipment 14 a onboardthe towing vessel 11.

References 15 a and 15 b each denotes a low frequency pressure sensor,that can acquire pressure data in the frequency range 0.03 to 1 Hz. Eachof these is located adjacent to one seismic receiver, the receivers 13 aand 13 b respectively. The low frequency pressure data acquired by thepressure sensors 15 a, 15 b are passed to second processing and/orrecording equipment 14 b on the survey vessel 11, which processes thelow frequency pressure data acquired by the pressure sensor 15 a toobtain the local sea height above the pressure sensor 15 a (which issubstantially equivalent to the local sea height above the receiver 13 aadjacent to the pressure sensor 15 a). Similarly the low frequencypressure data acquired by the pressure sensor 15 b are processed toobtain the local sea height above the pressure sensor 15 b (which issubstantially equivalent to the local sea height above the adjacentreceiver 13 b).

In practice the pressure sensors 15 a, 15 b will repeatedly sample thepressure in the frequency range of 0.03 to 1 Hz, so that thetime-varying local sea height above each sensor may be determined. Theresulting sea-height data may be used for any of the purposes describedabove—for example, the time-varying profile of the sea-surface may bedetermined from these local height measurements. (In practice, astreamer will contain many more low frequency pressure sensors thanshown in FIG. 8, so that more local sea-height measurements will beavailable for the determination of the sea profile.) As noted above, theinvention may be performed by using a seismic pressure sensor to obtainthe low frequency pressure data. This is illustrated in FIG. 8 byreference 13 c, which denotes a seismic receiver having a seismicpressure sensor for which the associated digital low-cut filter has beendisabled and which therefore that can acquire pressure data in theapproximate range of 0.03 to 1 Hz. The receiver 13 c therefore does notrequire a co-located low frequency pressure sensor. Low frequencypressure data acquired by the receiver 13 c are passed to the secondprocessing and/or recording equipment 14 b on the survey vessel 11, andseismic data acquired by the receiver 13 c are passed to the firstprocessing and/or recording apparatus 14 a. The low frequency pressuredata acquired by the receiver 13 c are processed to obtain the local seaheight above the receiver 13 c.

In practice, the invention is likely to be effected either by using lowfrequency pressure sensors substantially co-located with each seismicreceiver or by using each seismic pressure sensor to obtain lowfrequency pressure data by disabling the associated digital low-cutfilter. The two methods are both shown in FIG. 8 primarily for thepurposes of illustration although, in principle, these two method couldbe combined.

References 15 c denotes a pressure sensor provided on the source array10 and that can acquire pressure data in the approximate frequency range0.03 to 1 Hz. The low frequency pressure data acquired by the pressuresensor 15 c are also passed to the second processing and/or recordingequipment 14 b on the survey vessel 11, and may be processed to obtainthe local sea height above the pressure sensor 15 c (which issubstantially equivalent to the local sea height above the source array10).

The processing and/or recording apparatus 14 a, 14 b may be combined ina single processing and/or recording apparatus. They may comprise anapparatus 1 as shown in FIG. 7. The seismic data and low frequencypressure data may simply be recorded on the survey vessel for laterprocessing, or one or both may be processed in real-time or nearreal-time (for example to monitor the depth of the streamer).

1. A method of determining the height of the surface of a fluid column,the method comprising the steps of: providing a sensor within a fluidcolumn, the sensor being sensitive to pressure waves at frequenciesbelow about 1 Hz; using said sensor to receive and acquire pressure datain a frequency band comprised between about 0.03 Hz and about 1 Hz; andprocessing said pressure data to obtain information about the height ofthe surface of the fluid column above the sensor.
 2. A method accordingto claim 1, wherein the sensor is comprised in an array of seismicsensors.
 3. A method according to claim 1, wherein the sensor iscomprised in an instrumented cable.
 4. A method according to claim 2,wherein the sensor, is a seismic sensor.
 5. A method according to claim4, wherein the seismic sensor receives and acquires seismic datasubstantially simultaneously to the receiving and the acquiring of thepressure data in the frequency bank comprised between about 0.03 Hz andabout 1 Hz.
 6. A method according to claim 4, wherein the seismic sensoris a hydrophone.
 7. A method according to claim 3, wherein theinstrumented cable is a towed streamer comprising a plurality ofdecoupled sensors.
 8. A method according to claim 1, wherein the sensoris associated with a seismic source or with a respective seismic source.9. A method according to claim 1, further comprising the step ofcorrecting the acquired pressure data to take into account the movementof the sensor relative to the fluid column.
 10. A method according toclaim 1, wherein the step of processing said pressure data comprisesapplying the following correction filter to the pressure data acquiredby a sensor:p(ω)=ρ g h (ω) exp(−ω² z/g) where p(ω) is the pressure sensed by thesensor, ρ is the density of the fluid, g is the acceleration due togravity, z is the depth of the sensor below the Mean Sea Level, ω is theangular frequency of the surface wave and h is the upward displacementof the surface of the fluid column directly above the sensor andrelative to the Mean Sea Level.
 11. A method according to claim 1,wherein the step of processing said pressure data comprises applying acorrection filter to the data acquired by a sensor, said correctionfilter being a numerical combination of the following equations:p=ρ g h cosh(k(d−z))/cosh(kd)andω² =g k tanh (kd) where p is the pressure sensed by the sensor, ρ is thedensity of the water, g is the acceleration due to gravity, z is thedepth of said sensor below the Mean Sea Level, ω is the angularfrequency of the surface wave, d is the ocean depth relative to the MeanSea Level and h is the upward displacement of the sea surface directlyabove the sensor and relative to the Mean Sea Level.
 12. A method asclaimed in claim 1, wherein the step of processing said pressure datacomprises the steps of: obtaining information about the height of thesurface of the fluid column above each of a plurality of sensors; andgenerating a profile of the sea surface from the information about theheight of the surface of the fluid column above each of the plurality ofsensors.
 13. A method of processing seismic data, the method comprisingthe steps of: providing a first sensor within a fluid column, the firstsensor being sensitive to pressure waves at frequencies down to about0.03 Hz; providing a second sensor within the fluid column, the secondsensor being a seismic sensor; using said first sensor to receive andacquire pressure data in a frequency band comprised between about 0.03Hz and about 1 Hz; using said second sensor to receive and acquireseismic data substantially simultaneously with the step of receiving andacquiring the pressure data; processing said pressure data to obtaininformation about the height of the surface of the fluid column abovethe first sensor; and processing the seismic data using the informationabout the height of the surface of the fluid column above the firstsensor thereby to attenuate effects of a rough sea ghost reflection inthe processed seismic data.
 14. A method as claimed in claim 13 whereinthe first sensor is substantially co-located with the or a respectivesecond sensor.
 15. A method as claimed in claim 13 wherein the firstsensor is the second sensor.
 16. A method as claimed in claim 13,wherein the step of processing the seismic data comprises: computing areflection response by Kirchhoff integration; calculating adeconvolution operator; and applying said deconvolution operator to theseismic data.
 17. A system for determining the height of the surface ofa fluid column, the system comprising: a sensor within a fluid column,or the each sensor being adapted to, in use, receive and acquirepressure data in a frequency band comprised between about 0.03 Hz andabout 1 Hz; and processing apparatus for processing said pressure datato obtain information about the height of the surface of the fluidcolumn above the sensor.
 18. A system as claimed in claim 14 wherein theprocessing apparatus comprises a programmable data processor.
 19. A datacarrier containing a stored program for a programmable data processor ofa system as defined in claim
 15. 20. A computer programmed to perform amethod as defined in claim
 1. 21. A program for programming a computerto perform a method as defined in claim
 1. 22. A seismic surveyingarrangement comprising: a seismic source disposed within a fluid column;a first sensor disposed within the fluid column and spaced from theseismic source, the sensor being adapted to receive and acquire pressuredata in a frequency band comprised between about 0.03 Hz and about 1 Hz;a second sensor, the second sensor being adapted to receive and acquireseismic data substantially simultaneously with the acquisition of thepressure data; first processing apparatus for processing said pressuredata to obtain information about the height of the surface of the fluidcolumn above the first sensor; and second processing apparatus forprocessing the seismic data using the information about the height ofthe surface of the fluid column above the first sensor thereby toattenuate effects of a rough sea ghost reflection in the processedseismic data.
 23. A seismic surveying arrangement as claimed in claim 22wherein the first sensor is substantially co-located with the secondsensor.
 24. A seismic surveying arrangement as claimed in claim 22wherein the first sensor is the second sensor.
 25. A seismic surveyingarrangement as claimed in claim 22, wherein the first processingapparatus is the second processing apparatus.
 26. A seismic surveyingarrangement as claimed in claim 22 wherein the first processingapparatus comprises a programmable data processor.
 27. A data carriercontaining a stored program for a programmable data processor of aseismic surveying arrangement as defined in claim 26.